I am writing a media plugin that uses custom tables to store its contents. (eg not the post tables where wordpress stores it attachment data). Now i'm looking for a way to use the default wordpress comment system to add comments to it. (These comments will not be in the regular comment table but also a custom table.

I need two things:

A hook that allows me to intercept
the comment submit and process it
with my own code if criteria are
met. The criteria itself is
arbitrary and not important, just
need a way to get to the post data
before Wordpress processes it.

A filter that allows me to replace the wp_list_comments() data with my own tabledata.

It's a hacky idea, I know, but the post/attachment table is too limiting for what I need.
Any ideas?

and as for "intercept the comment submit and process" chips answer would be the best way using preprocess_comment filter hook but you want be able to avoid WordPress form inserting the comment to the default table as well, so you can use wp_insert_comment action hook to remove the comment from the default table right after its inserted:

function my_handling_function( $comment_data ) {
// Here, do whatever you need to do with the comment
// modify it, pull data out of it to save somewhere
// or whatever you need to do
// just be sure to return $comment_data when you're done!
return $comment_data;
}
add_filter( 'preprocess_comment', 'my_handling_function' );